Golf ball

ABSTRACT

At all points Pa in a zone A extending over a distance range from equal to or greater than 1 mm to less than 5 mm from the central point of a core  4,  a mathematical formula, Ha 2 −Ha 1 &lt;5, is satisfied. At any of points Pb in a zone B extending over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core  4 , a mathematical formula, Hb 2 −Hb 1 ≧5, is satisfied. Ha 1  and Ha 2  indicate JIS-C hardnesses at points Pa 1  and Pa 2  located radially inward and outward of each point Pa at a distance of 1 mm from the point Pa. Hb 1  and Hb 2  indicate JIS-C hardnesses at points Pb 1  and Pb 2  located radially inward and outward of the point Pb at a distance of 1 mm from the point Pb.

This application claims priority on Patent Application No. 2010-131641 filed in JAPAN on Jun. 9, 2010. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls including a center, an envelope layer, a mid layer and a cover.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball with excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.

An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.

Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.

In light of achieving various performance characteristics, golf balls each having a multilayer structure have been proposed. U.S. Pat. No. 6,468,169 (JP10-328326) discloses a golf ball including a core, an envelope layer, an inner cover and an outer cover. U.S. Pat. No. 6,271,296 (JP2001-17575) discloses a golf ball including a core, an envelope layer, a mid layer and a cover. U.S. Pat. No. 6,913,547 (JP2002-272880) discloses a golf ball including a core and a cover. The core consists of a center and an outer core layer. The cover consists of an inner cover layer and an outer cover layer. US2003/130066 (JP2003-135626) discloses a golf ball including a core and a cover. The core consists of a center and a mid layer. US2003/166422 (JP2003-205052) discloses a golf ball including a center, a mid layer and a cover. US2004/029648 (JP2004-130072) discloses a golf ball including a core and a cover. The core has a three-layer structure.

When a core that has an outer-hard/inner-soft structure and an excessive hardness distribution is hit with a driver, the energy loss is high in the core. The energy loss impairs resilience performance. When a core that has an outer-hard/inner-soft structure and an excessive hardness distribution is hit with a short iron, the spin rate is low. The low spin rate impairs controllability.

An object of the present invention is to provide a golf ball that provides a large flight distance when being hit with a driver and that has excellent controllability when being hit with a short iron.

SUMMARY OF THE INVENTION

A golf ball according to the present invention comprises a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer. The core comprises a center and an envelope layer positioned outside the center. A ratio of a volume of the core to a volume of a phantom sphere of the golf ball is equal to or greater than 76%. A JIS-C hardness Hc of the cover is less than JIS-C hardness Ho at a central point of the core. At all points Pa included in a zone A that extends over a distance range from equal to or greater than 1 mm to less than 5 mm from a central point of the core, the following mathematical formula (I) is satisfied. At any of points Pb included in a zone B that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core, the following mathematical formula (II) is satisfied.

Ha2−Ha1<5   (I),

Hb2−Hb1≧5   (II),

In the mathematical formula (I), Ha1 indicates a JIS-C hardness at a point Pa1 that is located radially inward of each point Pa at a distance of 1 mm from the point Pa, and Ha2 indicates a JIS-C hardness at a point Pa2 that is located radially outward of the point Pa at a distance of 1 mm from the point Pa. In the mathematical formula (II), Hb1 indicates a JIS-C hardness at a point Pb1 that is located radially inward of the point Pb at a distance of 1 mm from the point Pb, and Hb2 indicates a JIS-C hardness at a point Pb2 that is located radially outward of the point Pb at a distance of 1 mm from the point Pb.

In the golf ball according to the present invention, a hardness distribution of the core is appropriate. The core has a low energy loss when being hit with a driver. When the golf ball is hit with a driver, a large flight distance is obtained. The golf ball has excellent controllability when being hit with a short iron.

Preferably, the JIS-C hardness Hc of the cover is equal to or less than 65. Preferably, a thickness of the cover is equal to or less than 0.8 mm.

Preferably, a JIS-C hardness Hm of the mid layer is equal to or greater than 90. Preferably, a thickness of the mid layer is equal to or less than 1.5 mm.

The cover is formed from a resin composition. Preferably, a principal component of a base material of the resin composition is a thermoplastic polyurethane.

Preferably, a shear loss elastic modulus G″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., is equal to or less than 1.95×10⁷ Pa, and a ratio (E″/G″) of a tensile loss elastic modulus E″ of the resin composition, which is measured under the same conditions, to the shear loss elastic modulus G″ is equal to or greater than 1.76. Preferably, the tensile loss elastic modulus E″ is equal to or greater 2.00×10⁷ Pa.

Preferably, a polyol component of the thermoplastic polyurethane is polytetramethylene ether glycol having a number average molecular weight of 1500 or less.

Preferably, a difference between a JIS-C hardness He at a surface of the core and the hardness Hb2 is equal to or greater than 10. Preferably, a difference between a JIS-C hardness He at a surface of the core and the hardness Ho is equal to or less than 40.

Preferably, the hardness Ho is equal to or greater than 40 but equal to or less than 80. Preferably, a JIS-C hardness He at a surface of the core is equal to or greater than 75 but equal to or less than 95. Preferably, a difference between a JIS-C hardness He at a surface of the core and a JIS-C hardness Hi at an innermost portion of the envelope layer is equal to or greater than 10 but equal to or less than 25.

Preferably, a thickness of the envelope layer is equal to or greater than 8 mm but equal to or less than 18 mm. Preferably, a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm.

Preferably, a JIS-C hardness He at a surface of the core is greater than a JIS-C hardness at a surface of the center, and the hardness Hm of the mid layer is greater than the hardness He. Preferably, a difference between the hardness Ho and the hardness Hc is equal to or greater than 3.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to one embodiment of the present invention;

FIG. 2 is a graph showing a hardness distribution of each core of golf balls according to Examples 1 and 4 to 6 of the present invention;

FIG. 3 is a graph showing a hardness distribution of a core of a golf ball according to Example 2 of the present invention;

FIG. 4 is a graph showing a hardness distribution of a core of a golf ball according to Example 3 of the present invention;

FIG. 5 is a graph showing a hardness distribution of a core of a golf ball according to Example 7 of the present invention;

FIG. 6 is a graph showing a hardness distribution of a core of a golf ball according to Example 8 of the present invention;

FIG. 7 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 1;

FIG. 8 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 2;

FIG. 9 is a graph showing a hardness distribution of each core of golf balls according to Comparative Examples 3 and 4; and

FIG. 10 is a graph showing a hardness distribution of a core of a golf ball according to Comparative Example 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention based on preferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6 positioned outside the core 4, and a cover 8 positioned outside the mid layer 6. The core 4 includes a spherical center 10 and an envelope layer 12 positioned outside the center 10. On the surface of the cover 8, a large number of dimples 14 are formed. Of the surface of the golf ball 2, a part other than the dimples 14 is a land 16. The golf ball 2 includes a paint layer and a mark layer on the external side of the cover 8 although these layers are not shown in the drawing.

The golf ball 2 has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is preferably equal to or less than 44 mm and more preferably equal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is preferably equal to or greater than 44 g and more preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is preferably equal to or less than 45.93 g.

Preferably, the center 10 is obtained by crosslinking a rubber composition. Examples of preferable base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred that the polybutadiene is included as a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight. The proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% and more preferably equal to or greater than 80%.

The rubber composition of the center 10 includes a co-crosslinking agent. The co-crosslinking agent achieves high resilience of the center 10. Examples of preferable co-crosslinking agents in light of resilience performance include monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specific examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferred.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent is preferably equal to or greater than 5 parts by weight, and more preferably equal to or greater than 10 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the co-crosslinking agent is preferably equal to or less than 30 parts by weight, more preferably equal to or less than 25 parts by weight, and particularly preferably equal to or less than 20 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the center 10 includes an organic peroxide together with a co-crosslinking agent. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of the organic peroxide is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.2 parts by weight, and particularly preferably equal to or greater than 0.3 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic peroxide is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the center 10 includes an organic sulfur compound. Examples of preferable organic sulfur compounds include monosubstitutions such as diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(4-cyanophenyl)disulfide and the like; disubstitutions such as bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, bis(2-cyano-5-bromophenyl)disulfide and the like; trisubstitutions such as bis(2,4,6-trichlorophenyl)disulfide, bis(2-cyano-4-chloro-6-bromophenyl)disulfide and the like; tetrasubstitutions such as bis(2,3,5,6-tetrachlorophenyl)disulfide and the like; and pentasubstitutions such as bis(2,3,4,5,6-pentachlorophenyl)disulfide, bis(2,3,4,5,6-pentabromophenyl)disulfide and the like. The organic sulfur compound contributes to resilience performance. Particularly preferable organic sulfur compounds are diphenyl disulfide and bis(pentabromophenyl)disulfide.

In light of resilience performance of the golf ball 2, the amount of the organic sulfur compound is preferably equal to or greater than 0.1 parts by weight and more preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic sulfur compound is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the center 10. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the center 10 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, an vulcanization accelerator and the like are added to the rubber composition of the center 10. Crosslinked rubber powder or synthetic resin powder may be also dispersed in the rubber composition.

In light of resilience performance and durability, the center 10 has a central hardness Ho of preferably 40 or greater, more preferably 45 or greater, and particularly preferably 50 or greater. In light of suppression of spin, the central hardness Ho is preferably equal to or less than 80, more preferably equal to or less than 75, and particularly preferably equal to or less than 70. The central hardness Ho is measured by pressing a JIS-C type hardness scale against the central point of a cut plane of the center 10 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

The hardness of the center 10 gradually increases from its central point toward its surface. The center 10 has a surface hardness greater than the central hardness Ho.

The center 10 has a diameter of preferably 10 mm or greater but 20 mm or less. The center 10 having a diameter of 10 mm or greater can achieve excellent feel at impact. In this respect, the diameter is more preferably equal to or greater than 12 mm and particularly preferably equal to or greater than 13 mm. When the center 10 has a diameter of 20 mm or less, the envelope layer 12 having a sufficiently large thickness can be formed. In this respect, the diameter is more preferably equal to or less than 18 mm and particularly preferably equal to or less than 17 mm.

The envelope layer 12 is obtained by crosslinking a rubber composition. Examples of base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred that the polybutadiene is included as a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight. The proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% and more preferably equal to or greater than 80%.

In order to crosslink the envelope layer 12, a co-crosslinking agent is preferably used. Examples of preferable co-crosslinking agents in light of resilience performance include monovalent or bivalent metal salts of an α,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specific examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate and magnesium methacrylate. In light of resilience performance, zinc acrylate and zinc methacrylate are particularly preferred.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent is preferably equal to or greater than 20 parts by weight, more preferably equal to or greater than 25 parts by weight, and particularly preferably equal to or greater than 30 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the co-crosslinking agent is preferably equal to or less than 60 parts by weight, more preferably equal to or less than 50 parts by weight, and particularly preferably equal to or less than 45 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the envelope layer 12 includes an organic peroxide together with a co-crosslinking agent. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of the organic peroxide is preferably equal to or greater than 0.1 parts by weight, more preferably equal to or greater than 0.3 parts by weight, and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic peroxide is preferably equal to or less than 2.0 parts by weight, more preferably equal to or less than 1.5 parts by weight, and particularly preferably equal to or less than 1.0 parts by weight, per 100 parts by weight of the base rubber.

Preferably, the rubber composition of the envelope layer 12 includes an organic sulfur compound. The organic sulfur compounds described above for the center 10 can be used for the envelope layer 12. In light of resilience performance of the golf ball 2, the amount of the organic sulfur compound is preferably equal to or greater than 0.1 parts by weight and more preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber. In light of soft feel at impact, the amount of the organic sulfur compound is preferably equal to or less than 1.5 parts by weight, more preferably equal to or less than 1.0 parts by weight, and particularly preferably equal to or less than 0.8 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the envelope layer 12. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate and magnesium carbonate. Powder of a metal with a high specific gravity may be included as a filler. Specific examples of metals with a high specific gravity include tungsten and molybdenum. The amount of the filler is determined as appropriate so that the intended specific gravity of the envelope layer 12 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator. According to need, various additives such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, a dispersant and the like are included in the envelope layer 12 in an adequate amount. Crosslinked rubber powder or synthetic resin powder may be also included in the envelope layer 12.

During formation of the envelope layer 12, the center 10 is covered with two uncrosslinked or semi-crosslinked half shells. These half shells are compressed and heated. By this heating, a crosslinking reaction takes place to complete the envelope layer 12. The crosslinking temperature is generally equal to or higher than 140° C. but equal to or lower than 180° C. The time period for crosslinking the envelope layer 12 is generally equal to or longer than 10 minutes but equal to or shorter than 60 minutes.

The hardness of the envelope layer 12 gradually increases from its innermost portion to its surface. In light of resilience performance, a hardness He at the surface of the envelope layer 12 (namely, the surface of the core 4) is preferably equal to or greater than 75, more preferably equal to or greater than 80, and particularly preferably equal to or greater than 85. In light of feel at impact, the hardness He is preferably equal to or less than 95, more preferably equal to or less than 93, and particularly preferably equal to or less than 92. The hardness He is measured by pressing a JIS-C type hardness scale against the surface of the core 4. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

In light of suppression of spin, the difference (He−Hi) between the surface hardness He of the envelope layer 12 and a hardness Hi at the innermost portion of the envelope layer 12 is preferably equal to or greater than 10, more preferably equal to or greater than 12, and particularly preferably equal to or greater than 15. In light of ease of production and durability, the difference (He−Hi) is preferably equal to or less than 25.

The hardness Hi is measured for a hemisphere obtained by cutting the core 4. The hardness Hi is measured by pressing a JIS-C type hardness scale against the cut plane of the hemisphere. The hardness scale is pressed against a region surrounded by a first circle and a second circle. The first circle is the boundary between the center 10 and the envelope layer 12. The second circle is concentric with the first circle and has a radius larger than the radius of the first circle by 1 mm. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

The envelope layer 12 has a thickness of preferably 8 mm or greater but 18 mm or less. The envelope layer 12 having a thickness of 8 mm or greater can suppress spin. In this respect, the thickness is more preferably equal to or greater than 9 mm and particularly preferably equal to or greater than 10 mm. When the envelope layer 12 has a thickness of 18 mm or less, the center 10 having a large diameter can be formed. The center 10 having a large diameter can suppress spin. In this respect, the thickness is more preferably equal to or less than 16 mm and particularly preferably equal to or less than 15 mm.

In light of suppression of spin, the difference (He−Ho) between the surface hardness He of the core 4 and the central hardness Ho of the center 10 is preferably equal to or greater than 20 and particularly preferably equal to or greater than 25. In light of resilience performance of the core 4, the difference (He−Ho) is preferably equal to or less than 40 and particularly preferably equal to or less than 35.

In the present specification, a zone that extends over a distance range from equal to or greater than 1 mm to less than 5 mm from the central point of the core 4 is referred to as “zone A”, and a zone that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core 4 is referred to as “zone B”.

At all points Pa included in the zone A, the following mathematical formula (I) is satisfied.

Ha2−Ha1<5   (I)

In the mathematical formula (I), Ha1 indicates the JIS-C hardness at a point Pa1. The point Pa1 is located radially inward of each point Pa. The distance from the point Pa to the point Pa1 is 1 mm. In the mathematical formula (I), Ha2 indicates the JIS-C hardness at a point Pa2. The point Pa2 is located radially outward of each point Pa. The distance from the point Pa to the point Pa2 is 1 mm. The hardnesses Ha1 and Ha2 are measured by pressing a JIS-C type hardness scale against a cut plane of the center 10 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

The core 4 that satisfies the mathematical formula (I) has a low energy loss when being hit with a golf club. The core 4 can achieve high resilience of the golf ball 2. The golf ball 2 having the core 4 has excellent flight performance. In light of flight performance, the difference (Ha2−Ha1) is preferably equal to or less than 4 and particularly preferably equal to or less than 3. The difference (Ha2−Ha1) may be zero.

At any of points Pb included in the zone B, the following mathematical formula (II) is satisfied.

Hb2−Hb1≧5   (II)

In the mathematical formula (II), Hb1 indicates the JIS-C hardness at a point Pb1. The point Pb1 is located radially inward of the point Pb. The distance from the point Pb to the point Pb1 is 1 mm. In the mathematical formula (II), Hb2 indicates the JIS-C hardness at a point Pb2. The point Pb2 is located radially outward of the point Pb. The distance from the point Pb to the point Pb2 is 1 mm. The hardnesses Hb1 and Hb2 are measured by pressing a JIS-C type hardness scale against a cut plane of the center 10 that has been cut into two halves. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

The core 4 that satisfies the mathematical formula (II) suppresses spin of the golf ball 2. In this respect, the difference (Hb2−Hb1) is particularly preferably equal to or greater than 7. In light of reduced energy loss upon hitting with a golf club, the difference (Hb2−Hb1) is preferably equal to or less than 20 and particularly preferably equal to or less than 15.

The ratio of the volume of the core 4 to the volume of a phantom sphere of the golf ball 2 is equal to or greater than 76%. In other words, the core 4 is large. The core 4 can achieve excellent resilience performance of the golf ball 2. The core 4 can suppress spin of the golf ball 2. In this respect, the ratio is more preferably equal to or greater than 78% and particularly preferably equal to or greater than 80%. The surface of the phantom sphere is the surface of the golf ball 2 when it is postulated that no dimple 14 exists.

In light of suppression of spin, the difference (He−Hb2) between the surface hardness He of the core 4 and the hardness Hb2 is preferably equal to or greater than 10, more preferably equal to or greater than 12, and particularly preferably equal to or greater than 15.

A resin composition is suitably used for the mid layer 6. Examples of the base polymer of the resin composition include ionomer resins, styrene block-containing thermoplastic elastomers, thermoplastic polyester elastomers, thermoplastic polyamide elastomers and thermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. Ionomer resins are highly elastic. As described later, the cover 8 of the golf ball 2 is thin and flexible. Thus, when the golf ball 2 is hit with a driver, the mid layer 6 significantly deforms. The mid layer 6 including an ionomer resin contributes to resilience performance upon a shot with a driver. An ionomer resin and another resin may be used in combination. In this case, in light of resilience performance, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an α-olefin, and 10% by weight or more but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an α-olefin, 5% by weight or more but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid or methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan MK7320” and “Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000” and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000” and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation.

Two or more ionomer resins may be used in combination for the mid layer 6. An ionomer resin neutralized with a monovalent metal ion, and an ionomer resin neutralized with a bivalent metal ion may be used in combination.

The mid layer 6may include a highly elastic resin. Examples of highly elastic resins include polybutylene terephthalate, polyphenylene ether, polyethylene terephthalate, polysulfone, polyethersulfone, polyphenylene sulfide, polyarylate, polyamide imide, polyetherimide, polyether ether ketone, polyimide, polytetrafluoroethylene, polyamino bismaleimide, polybisamide triazole, polyphenylene oxide, polyacetal, polycarbonate, acrylonitrile-butadiene-styrene copolymers and acrylonitrile-styrene copolymers.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener and the like are included in the mid layer 6 in an adequate amount. For forming the mid layer 6, known methods such as injection molding, compression molding and the like can be used.

The mid layer 6 has a hardness Hm of preferably 90 or greater. The mid layer 6 having a hardness Hm of 90 or greater achieves excellent resilience performance of the golf ball 2. The mid layer 6 having a hardness Hm of 90 or greater can achieve an outer-hard/inner-soft structure of the sphere consisting of the core 4 and the mid layer 6. The sphere having the outer-hard/inner-soft structure suppress spin of the golf ball 2. In these respects, the hardness Hm is particularly preferably equal to or greater than 92. In light of feel at impact, the hardness Hm is equal to or less than 98 and particularly preferably equal to or less than 97. In light of suppression of spin, preferably, the hardness Hm of the mid layer 6 is greater than the surface hardness He of the core 4, and the surface hardness He of the core 4 is greater than the surface hardness of the center 10.

The hardness Hm is measured with an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.) to which a JIS-C type spring hardness scale is mounted. For the measurement, a slab formed by hot press and having a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from a resin composition that is the same as the resin composition of the mid layer 6 is used for the measurement.

In light of suppression of spin, the mid layer 6 has a thickness of preferably 0.3 mm or greater, more preferably 0.5 mm or greater, and particularly preferably 0.6 mm or greater. In light of feel at impact, the thickness is preferably equal to or less than 1.5 mm, more preferably equal to or less than 1.2 mm, and particularly preferably equal to or less than 1.0 mm.

The cover 8 is formed from a resin composition. Examples of the base polymer of the resin composition include polyurethanes, polyesters, polyamides, polyolefins, polystyrenes and ionomer resins. Particularly, polyurethanes are preferred. Polyurethanes are flexible. When the golf ball 2 with the cover 8 including a polyurethane is hit with a short iron, the spin rate is high. The cover 8 formed from a polyurethane contributes to the controllability upon a shot with a short iron. The polyurethane also contributes to the scuff resistance of the cover 8.

When the golf ball 2 is hit with a driver, a long iron or a middle iron, the sphere consisting of the core 4 and the mid layer 6 becomes significantly distorted since the head speed is high. Since this sphere has an outer-hard/inner-soft structure, the spin rate is suppressed. The suppression of the spin rate achieves a large flight distance. When the golf ball 2 is hit with a short iron, this sphere becomes less distorted since the head speed is low. When the golf ball 2 is hit with a short iron, the behavior of the golf ball 2 mainly depends on the cover 8. Since the cover 8 including the polyurethane is flexible, a high spin rate is obtained. The high spin rate achieves excellent controllability. In the golf ball 2, both desired flight performance upon shots with a driver, a long iron, and a middle iron and desired controllability upon a shot with a short iron are achieved.

When the golf ball 2 is hit, the cover 8 including the polyurethane absorbs the shock. This absorption achieves soft feel at impact. Particularly, when the golf ball 2 is hit with a short iron or a putter, the cover 8 achieves excellent feel at impact.

When being hit, compressive stress is applied to the cover 8 due to movement of the head of a golf club. Since the face of the golf club has a loft angle, shear stress is also applied to the cover 8 when being hit. The head speed of a short iron is low, and the loft angle of a short iron is high. Thus, when the golf ball 2 is hit with a short iron, the shear stress greatly influences the deformation behavior of the cover 8. The head speed of a driver is high, and the loft angle of a driver is low. Thus, when the golf ball 2 is hit with a driver, the compressive stress greatly influences the deformation behavior of the cover 8.

The cover 8 has a shear loss elastic modulus G″ of preferably 1.95×10⁷ Pa or less. As described above, when being hit with a short iron, the deformation behavior of the cover 8 is greatly influenced by the shear stress. The spin rate obtained when being hit with a short iron correlates with the shear loss elastic modulus G″. When the golf ball 2 with the cover 8 having a shear loss elastic modulus G″ of 1.95×10⁷ Pa or less is hit with a short iron, the spin rate is high. The cover 8 can achieve excellent controllability. In this respect, the shear loss elastic modulus G″ is particularly equal to or less than 1.83×10⁷ Pa. In light of ease of forming the cover 8, the shear loss elastic modulus G″ is preferably equal to or greater than 1.00×10⁶ Pa and particularly equal to or greater than 1.10×10⁶ Pa.

The ratio (E″/ G″) of a tensile loss elastic modulus E″ of the cover 8 to the shear loss elastic modulus G″ is preferably equal to or greater than 1.76. As described above, when being hit with a driver, the deformation behavior of the cover 8 is greatly influenced by the compressive stress. The spin rate obtained when being hit with a driver correlates with the tensile loss elastic modulus E″. When the golf ball 2 with the cover 8 having a ratio (E″/ G″) of 1.76 or greater is hit with a driver, the spin rate is low, and when the golf ball 2 is hit with a short iron, the spin rate is high. In this respect, the ratio (E″/ G″) is more preferably equal to or greater than 1.86 and particularly preferably equal to or greater than 1.90. In light of ease of forming the cover 8, the ratio (E″/ G″) is preferably equal to or less than 6.0 and particularly preferably equal to or less than 5.5.

The tensile loss elastic modulus E″ is preferably equal to or greater than 2.00×10⁷ Pa, more preferably equal to or greater than 2.20×10⁷ Pa, and particularly preferably equal to or greater than 2.40×10⁷ Pa. The tensile loss elastic modulus E″ is preferably equal to or less than 1.00×10⁸ Pa.

The shear loss elastic modulus G″ and the tensile loss elastic modulus E″ can be controlled by adjusting the molecular weight of a polyol, the molecular weight of a polyisocyanate, a ratio (NCO/OH), and the like.

For measuring the shear loss elastic modulus G″, a sheet having a thickness of 2 mm is obtained by press molding from a resin composition that is the same as the resin composition of the cover 8. A test piece having a width of 10 mm and an inter-clamp distance of 10 mm is punched out from the sheet. The shear loss elastic modulus G″ is measured for the test piece. The measurement conditions are as follows.

Apparatus: “Rheometer ARES”, manufactured by TA instruments

Measurement mode: twisting (shearing)

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1%

For measuring the tensile loss elastic modulus E″, a sheet having a thickness of 2 mm is obtained by press molding from a resin composition that is the same as the resin composition of the cover 8. A test piece having a width of 4 mm and an inter-clamp distance of 20 mm is punched out from the sheet. The tensile loss elastic modulus E″ is measured for the test piece. The measurement conditions are as follows.

Apparatus: the dynamic viscoelasticity measuring apparatus “Rheogel-E4000”, manufactured by UBM

Measurement mode: pulling

Measurement temperature: 0° C.

Vibration frequency: 10 Hz

Measurement distortion: 0.1%

A time for which a golf ball and a club contact each other is several hundred microseconds. Thus, the frequency of deformation of the golf ball 2 when being hit is several thousand Hz. On average, the golf ball 2 is hit at substantially normal temperature (25° C.). On the basis of a general time conversion rule of polyurethane, a deformation having a frequency of several thousand Hz in the environment having a temperature of 25° C. corresponds to a deformation having a frequency of 10 Hz in the environment having a temperature of 0° C. Thus, in the present invention, the shear loss elastic modulus G″ and the tensile loss elastic modulus E″ are measured under the conditions of a vibration frequency of 10 Hz and a temperature of 0° C.

A polyurethane and another resin may be used in combination for the cover 8. In this case, in light of spin performance and feel at impact, the polyurethane is included as the principal component of the base polymer. The proportion of the polyurethane to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 70% by weight, and particularly preferably equal to or greater than 85% by weight.

For the cover 8, thermoplastic polyurethanes and thermosetting polyurethanes can be used. In light of productivity, thermoplastic polyurethanes are preferred. A thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment.

The polyurethane includes a polyol component. A polymeric polyol is preferred. Specific examples of polymeric polyols include polyetherpolyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG) and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA) and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination.

Particularly, polytetramethylene ether glycol is preferred. The spin rate obtained when the golf ball 2 is hit with a short iron has a high correlation with the content of polytetramethylene ether glycol. Meanwhile, the spin rate obtained when the golf ball 2 is hit with a driver has a low correlation with the content of polytetramethylene ether glycol. The golf ball 2 including a polyurethane that includes polytetramethylene ether glycol in an appropriate amount has both excellent flight performance when being hit with a driver and excellent controllability when being hit with a short iron.

In light of controllability, the polyol has a number average molecular weight of preferably 200 or greater, more preferably 400 or greater, and particularly preferably 650 or greater. In light of suppression of spin, the molecular weight is preferably equal to or less than 1500, more preferably equal to or less than 1200, and particularly preferably equal to or less than 850.

The number average molecular weight is measured by gel permeation chromatography. The measurement conditions are as follows.

Apparatus: HLC-8120GPC (manufactured by Tosoh Corporation)

Eluant: tetrahydrofuran

Concentration: 0.2% by weight

Temperature: 40° C.

Column: TSK gel Super HM-M (manufactured by Tosoh Corporation)

Sample volume: 5 microliters

Flow rate: 0.5 milliliter/min

Reference material: polystyrene (“PStQuick Kit-H” manufactured by Tosoh Corporation)

The polymeric polyol component has a hydroxyl value of preferably 94 mg KOH/g or greater and particularly preferably 112 mg KOH/g or greater. The hydroxyl value is preferably equal to or less than 561 mg KOH/g and particularly preferably equal to or less than 173 mg KOH/g.

Examples of an isocyanate component in the polyurethane include aromatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, a mixture (TDI) of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylene diisocyanate (TMXDI) and paraphenylene diisocyanate (PPDI); and alicyclic polyisocyanates such as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenated xylylene diisocyanate (H₆XDI) and isophorone diisocyanate (IPDI). Two or more polyisocyanates may be used in combination. In light of weather resistance, TMXDI, XDI, HDI, H₆XDI, IPDI and H₁₂MDI are preferred, and H₁₂MDI is particularly preferred.

The polyurethane may include a chain extender as its component. Examples of chain extenders include low-molecular-weight polyols and low-molecular-weight polyamines.

Examples of low-molecular-weight polyols include diols, triols, tetraols and hexaols. Specific examples of diols include ethylene glycol, diethylene glycol, propanediol, dipropylene glycol, butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol and octanediol. Specific examples of triols include glycerin, trimethylolpropane and hexanetriol. Specific examples of tetraols include pentaerythritol and sorbitol. 1,4-butanediol is preferred.

Examples of low-molecular-weight polyamines include aliphatic polyamines, monocyclic aromatic polyamines and polycyclic aromatic polyamines. Specific examples of aliphatic polyamines include ethylenediamine, propylenediamine, butylenediamine and hexamethylenediamine. Specific examples of monocyclic aromatic polyamines include phenylenediamine, toluene diamine, dimethyl toluene diamine, dimethylthio toluene diamine and xylylenediamine.

The chain extender has a number average molecular weight of preferably 30 or greater, more preferably 40 or greater, and particularly 45 or greater. The molecular weight is preferably equal to or less than 400, more preferably equal to or less than 350, and particularly preferably equal to or less than 200. Low-molecular-weight polyols and low-molecular-weight polyamines that are used as chain extenders are low-molecular-weight compounds that almost do not have a molecular weight distribution. Thus, the low-molecular-weight polyols and the low-molecular-weight polyamines can be distinguished from the polymeric polyol.

The cover 8 may be formed from a composition including a thermoplastic polyurethane and an isocyanate compound. During or after forming the cover 8, the polyurethane is crosslinked with the isocyanate compound.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener and the like are included in the cover 8 in an adequate amount.

The cover 8 has a JIS-C hardness Hc of 65 or less. Use of the flexible cover 8 can achieve excellent controllability upon a shot with a short iron. In light of controllability, the hardness Hc is more preferably equal to or less than 60, even more preferably equal to or less than 55, and particularly preferably equal to or less than 50. If the hardness Hc is excessively low, the flight performance upon a shot with a driver is insufficient. In this respect, the hardness Hc is preferably equal to or greater than 20, more preferably equal to or greater than 25, and particularly preferably equal to or greater than 35. For measuring the hardness Hc, a slab formed from a resin composition that is the same as the resin composition of the cover 8 is used. The measurement method is the same as the measurement method for the hardness Hm of the mid layer 6.

The hardness Hc of the cover 8 is less than the central hardness Ho of the core 4. The golf ball 2 has excellent controllability upon a shot with a short iron. In light of controllability, the difference (Ho−Hc) is preferably equal to or greater than 3, more preferably equal to or greater than 5, and particularly preferably equal to or greater than 8. The difference (Ho−Hc) is preferably equal to or less than 15.

In light of flight performance upon a shot with a driver, the cover 8 has a thickness of preferably 0.8 mm or less, more preferably 0.6 mm or less, even more preferably 0.5 mm or less, and particularly preferably 0.4 mm or less. In light of controllability upon a shot with a short iron, the thickness is preferably equal to or greater than 0.10 mm and particularly preferably equal to or greater than 0.15 mm.

For forming the cover 8, known methods such as injection molding, compression molding and the like can be used. When forming the cover 8, the dimples 14 are formed by pimples formed on the cavity face of a mold.

In light of feel at impact, the golf ball 2 has an amount of compressive deformation of preferably 2.0 mm or greater, more preferably 2.1 mm or greater, and particularly preferably 2.2 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 3.5 mm, more preferably equal to or less than 3.0 mm, and particularly preferably equal to or less than 2.6 mm.

At measurement of the amount of compressive deformation, first, the golf ball 2 is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the golf ball 2. The golf ball 2, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1274 N is applied thereto, is measured.

The golf ball 2 may include a reinforcing layer between the mid layer 6 and the cover 8. The reinforcing layer firmly adheres to the mid layer 6 and also to the cover 8. The reinforcing layer suppresses separation of the cover 8 from the mid layer 6. As described above, the cover 8 of the golf ball 2 is thin. When the golf ball 2 is hit by the edge of a clubface, a wrinkle is likely to occur. The reinforcing layer suppresses occurrence of a wrinkle.

As the base polymer of the reinforcing layer, a two-component curing type thermosetting resin is suitably used. Examples of two-component curing type thermosetting resins include epoxy resins, urethane resins, acrylic resins, polyester resins, and cellulose resins. In light of strength and durability of the reinforcing layer, two-component curing type epoxy resins and two-component curing type urethane resins are preferred.

The reinforcing layer may include additives such as a coloring agent (typically, titanium dioxide), a phosphate-based stabilizer, an antioxidant, a light stabilizer, a fluorescent brightener, an ultraviolet absorber, an anti-blocking agent and the like. The additives may be added to the base material of the two-component curing type thermosetting resin, or may be added to the curing agent of the two-component curing type thermosetting resin.

The reinforcing layer is obtained by applying, to the surface of the mid layer 6, a liquid that is prepared by dissolving or dispersing the base material and the curing agent in a solvent. In light of workability, application with a spray gun is preferred. After the application, the solvent is volatilized to permit a reaction of the base material with the curing agent, thereby forming the reinforcing layer.

In light of suppression of a wrinkle, the reinforcing layer has a thickness of preferably 3 μm or greater and more preferably 5 μm or greater. In light of ease of forming the reinforcing layer, the thickness is preferably equal to or less than 300 μm, more preferably equal to or less than 50 μm, and particularly preferably equal to or less than 20 μm. The thickness is measured by observing a cross section of the golf ball 2 with a microscope. When the mid layer 6 has concavities and convexities on its surface from surface roughening, the thickness of the reinforcing layer is measured at a convex part.

In light of suppression of a wrinkle, the reinforcing layer has a pencil hardness of preferably 4 B or greater and more preferably B or greater. In light of reduced loss of the power transmission from the cover 8 to the mid layer 6 upon hitting the golf ball 2, the pencil hardness of the reinforcing layer is preferably equal to or less than 3 H. The pencil hardness is measured according to the standard of “JIS K5400”.

EXAMPLES

[Synthesis of Polyurethane #1]

Dicyclohexylmethane diisocyanate polytetramethylene ether glycol (PTMG) having a number average molecular weight of 1500, and 1,4-butanediol (BD) were prepared. H₁₂MDI and PTMG were heated to 80° C., and PTMG was put into a container containing H₁₂MDI, to obtain a mixed liquid. Dibutyltin dilaurate (manufactured by Aldrich, Inc.) was put into the container. Then, the mixed liquid was stirred at a temperature of 80° C. for 2 hours under a blanket of nitrogen gas. Further, BD heated to 80° C. was put into the container. The mixed liquid was stirred at a temperature of 80° C. for 1 minute under a blanket of nitrogen gas. The mixed liquid was cooled to room temperature. The mixed liquid was depressurized for 1 minute. By the depressurization, the mixed liquid was deaerated. After the deaeration, the mixed liquid was spread out in another container, and kept at 110° C. for 6 hours under a blanket of nitrogen gas. By the keeping, a urethane reaction took place, thereby obtaining a polyurethane #1. The polyurethane #1 had a JIS-C hardness of 45. Details of the materials are as follows.

H₁₂MDI: manufactured by Sumika Bayer Urethane Co. Ltd.

PTMG: trade name “PTMG-1500SN”, manufactured by Hodogaya Chemical Co., Ltd.

BD: manufactured by Wako Pure Chemical Industries, Ltd. The mole ratio of H₁₂MDI, PTMG, and BD was (3.39:1.00:2.39). The amount of dibutyltin dilaurate per 100 parts by weight of the total amount of H₁₂MDI, PTMG and BD was 0.005 parts by weight.

[Synthesis of Polyurethane #2]

A polyurethane #2 was synthesized in the same manner as the synthesis of the polyurethane #1, except the mole ratio of H₁₂MDI, PTMG and BD was (3.61:1.00:2.61). The polyurethane #2 had a JIS-C hardness of 47.

[Synthesis of Polyurethane #3]

A polyurethane #3 was synthesized in the same manner as the synthesis of the polyurethane #1, except PTMG having a number average molecular weight of 1000 (trade name “PTMG-1000SN”, manufactured by Hodogaya Chemical Co., Ltd.) was used and the mole ratio of H₁₂MDI, PTMG and BD was (2.63:1.00:1.63). The polyurethane #3 had a JIS-C hardness of 44.

Example 1

A rubber composition (1) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 20 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.7 parts by weight of dicumyl peroxide. The rubber composition (1) was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 15 minutes to obtain a center with a diameter of 15 mm.

A rubber composition (3) was obtained by kneading 100 parts by weight of a high-cis polybutadiene (the aforementioned “BR-730”), 42 parts by weight of zinc diacrylate, 5 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.5 parts by weight of diphenyl disulfide, and 0.7 parts by weight of dicumyl peroxide. Half shells were formed from the rubber composition (3). The center was covered with two half shells. The center and the half shells were placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 20 minutes to obtain a core with a diameter of 39.7 mm. An envelope layer was formed from the rubber composition (3). The amount of barium sulfate was adjusted such that the specific gravity of the envelope layer agrees with the specific gravity of the center and the weight of a golf ball is 45.4 g.

A resin composition (a) was obtained by kneading 50 parts by weight of an ionomer resin (the aforementioned “Surlyn 8945”) and 50 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”) with a twin-screw kneading extruder. The core was placed into a mold including upper and lower mold halves each having a hemispherical cavity. The core was covered with the resin composition (a) by injection molding to form a mid layer with a thickness of 1.0 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared. The base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type solid epoxy resin and 70 parts by weight of a solvent. The curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium oxide. The weight ratio of the base material liquid to the curing agent liquid is 1/1. This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 40° C. for 24 hours to obtain a reinforcing layer with a thickness of 10 μm.

A resin composition (b) was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (trade name “Elastollan XNY85A”, manufactured by BASF Japan Ltd.) and 4 parts by weight of titanium dioxide with a twin-screw kneading extruder. Half shells were obtained from the resin composition (b) by compression molding. The sphere consisting of the core, the mid layer and the reinforcing layer was covered with two of these half shells. The sphere and the half shells were placed into a final mold that includes upper and lower mold halves each having a hemispherical cavity and that has a large number of pimples on its cavity face. A cover was obtained by compression molding. The cover had a thickness of 0.5 mm. Dimples having a shape that was the inverted shape of the pimples were formed on the cover. A clear paint including a two-component curing type polyurethane as a base material was applied to this cover to obtain a golf ball of Example 1 with a diameter of 42.7 mm. A hardness distribution of the core of this golf ball is shown in Table 3.

Examples 2 to 8 and Comparative Examples 1 to 5]

Golf balls of Examples 2 to 8 and Comparative Examples 1 to 5 were obtained in the same manner as Example 1, except the specifications of the center, the envelope layer, the mid layer and the cover were as shown in Tables 6 to 9 below. The rubber composition of the core is shown in detail in Table 1 below. The resin compositions of the mid layer and the cover are shown in detail in Table 2 below. A hardness distribution of the core is shown in Tables 3 to 5. The golf ball according to Comparative Example 1 does not have an envelope layer.

[Shot with Driver (W #1)]

A driver with a titanium head (trade name “SRIXON W505”, manufactured by SRI Sports Limited, shaft hardness: X, loft angle: 8.5°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 50m/sec. The ball speed immediately after the hit and the distance from the launch point to the stop point were measured. The average value of data obtained by 12 measurements is shown in Tables 6 to 9 below.

[Shot with Short Iron]

A sand wedge (SW) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 21 m/sec, and the spin rate was measured immediately after the hit. The average value of data obtained by 12 measurements is shown in Tables 6 to 9 below. In addition, water was applied to a clubface and a golf ball, and the golf ball was hit. The spin rate was measured immediately after the hit. The average value of data obtained by 12 measurements is shown in Tables 6 to 9 below.

[Feel at Impact]

Ten golf players hit golf balls with sand wedges, and were asked about feel at impact. The evaluation was categorized as follows on the basis of the number of golf players who answered, “the feel at impact was excellent”.

A: 8 or more

B: 6 to 7

C: 4 to 5

D: 3 or less

The results are shown in Tables 6 to 9 below.

TABLE 1 Composition of Core (parts by weight) (1) (2) (3) (4) (5) (6) (7) (8) (9) BR-730 100 100 100 100 100 100 100 100 100 Zinc acrylate 20 38 42 45 39 22 37 22 23 Zinc oxide 5 5 5 5 5 5 5 5 5 Barium sulfate * * * * * * * * * Diphenyl disulfide 0.5 0.5 0.5 0.5 0.5 0.5 0.5 — 0.5 Dicumyl peroxide 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 Thionaphthol — — — — — — — 0.5 — Sulfur — — — — — — — — 0.05 * Appropriate amount

TABLE 2 Compositions of Mid Layer and Cover (parts by weight) (a) (b) (c) (d) (e) (f) (g) Surlyn 8945 50 — — — — — — Himilan AM7329 50 — — — — — — Elastollan XNY85A — 100 — — — — — Elastollan XNY90A — — 100 — — — — Elastollan XNY97A — — — 100 — — — Polyurethane #1 — — — — 100 — — Polyurethane #2 — — — — — 100 — Polyurethane #3 — — — — — — 100 Titanium dioxide — 4 4 4 4 4 4 Molecular weight of PTMG — 1800 1800 1800 1500 1500 1000 E″ (×10⁷ Pa) — 3.24 4.61 7.57 1.80 3.51 6.26 G″ (×10⁷ Pa) — 2.77 3.23 4.01 1.02 1.42 3.00 E″/G″ — 1.17 1.43 1.89 1.76 2.47 2.09 Hardness (JIS-C) 94 47 56 67 45 47 44 Hardness (Shore D) 64 32 38 47 30 32 29

Elastollan XNY85A, Elastollan XNY90A, Elastollan XNY97A, polyurethane #1, polyurethane #2 and polyurethane #3 are thermoplastic polyurethane elastomers in each of which a polyol component is polytetramethylene ether glycol. The number average molecular weight of each polytetramethylene ether glycol is as follows.

Elastollan XNY85A: 1800 Elastollan XNY90A: 1800

Elastollan XNY97A: 1800 Polyurethane #1: 1500

Polyurethane #2: 1500 Polyurethane #3: 1000

TABLE 3 Hardness Distribution of Core (JIS-C) Distance from central point (mm) Example 1 Example 2 Example 3 Example 4 0 60 60 60 60 1.0 61 60.8 61 61 2.0 62 61.6 62 62 3.0 63 62.4 63 63 4.0 64 63.2 64 64 5.0 65 64 65 65 6.0 65.7 65 65.7 65.7 7.0 66.5 66 66 66.5 8.0 74.5 67 74.5 74.5 9.0 75.7 68.76 75.7 75.7 10.0 77 77 77 77 11.0 77.8 77.8 77.8 77.8 12.0 78.6 78.6 78.6 78.6 13.0 79.4 79.4 79.4 79.4 14.0 80.2 80.2 80.2 80.2 15.0 81 81 81 81

TABLE 4 Hardness Distribution of Core (JIS-C) Distance from central point (mm) Example 5 Example 6 Example 7 Example 8 0 60 60 54 62 1.0 61 61 56 63 2.0 62 62 58 64 3.0 63 63 60 65 4.0 64 64 62 66 5.0 65 65 64 67 6.0 65.7 65.7 66 67.7 7.0 66.5 66.5 68 68.5 8.0 74.5 74.5 74.5 73.5 9.0 75.7 75.7 75.7 74.7 10.0 77 77 77 76 11.0 77.8 77.8 77.8 76.8 12.0 78.6 78.6 78.6 77.6 13.0 79.4 79.4 79.4 78.4 14.0 80.2 80.2 80.2 79.2 15.0 81 81 81 80

TABLE 5 Hardness Distribution of Core (JIS-C) Distance from central point Compa. Compa. Compa. Compa. Compa. (mm) Example 1 Example 2 Example 3 Example 4 Example 5 0 70 60 60 60 54 1.0 71.2 60.8 61 61 60 2.0 72.4 61.6 62 62 61.2 3.0 73.6 62.4 63 63 62.5 4.0 74.8 63.2 64 64 63.8 5.0 76 64 65 65 65 6.0 76 65 65.5 65.5 65.7 7.0 76 66 66 66 66.5 8.0 76 67 73 73 74.5 9.0 76 68 73.7 73.7 75.7 10.0 76 68.8 75 75 77 11.0 76.8 69.6 75.8 75.8 77.8 12.0 77.6 70.4 76.6 76.6 78.6 13.0 78.4 81.4 77.4 77.4 79.4 14.0 79.2 82.2 78.2 78.2 80.2 15.0 80 83 79 79 81

TABLE 6 Results of Evaluation Example 1 Example 2 Example 3 Center Composition (1) (1) (1) Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 15 15 15 Diameter (mm) 15 18 15 Envelope Composition (3) (3) (3) layer Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 20 20 20 Core Diameter (mm) 39.7 40.1 40.3 Volume ratio (%) 80.4 82.8 84.1 Central hardness Ho 60 60 60 (JIS-C) Surface hardness He 88 88 88 (JIS-C) Hardness distribution FIG. 2 FIG. 3 FIG. 4 Mid Composition (a) (a) (a) layer Hardness (JIS-C) 94 94 94 Thickness (mm) 1.0 1.0 0.9 Cover Composition (b) (c) (b) Hardness (JIS-C) 47 56 47 Thickness (mm) 0.5 0.3 0.3 Molecular weight of 1800 1800 1800 PTMG E″ (×10⁷ Pa) 3.24 4.61 3.24 G″ (×10⁷ Pa) 2.77 3.23 2.77 E″/G″ 1.17 1.43 1.17 Ball Amount of compressive 2.40 2.45 2.40 deformation (mm) Ha2-Ha1 (maximum value) 2.0 1.6 2.0 Hb2-Hb1 (maximum value) 9.0 10.0 9.0 W #1 Ball speed (m/s) 73.9 74.0 74.1 Spin (rpm) 2440 2310 2410 Flight distance (m) 248.5 251.0 250.0 SW Spin Dry (rpm) 6720 6530 6690 Spin Wet (rpm) 4500 3900 4470 Feel at impact A B A

TABLE 7 Results of Evaluation Example 4 Example 5 Example 6 Center Composition (1) (1) (1) Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 15 15 15 Diameter (mm) 15 15 15 Envelope Composition (3) (3) (3) layer Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 20 20 20 Core Diameter (mm) 39.7 39.7 39.7 Volume ratio (%) 80.4 80.4 80.4 Central hardness Ho 60 60 60 (JIS-C) Surface hardness He 88 88 88 (JIS-C) Hardness distribution FIG. 2 FIG. 2 FIG. 2 Mid Composition (a) (a) (a) layer Hardness (JIS-C) 94 94 94 Thickness (mm) 1.0 1.0 1.0 Cover Composition (e) (f) (g) Hardness (JIS-C) 45 47 44 Thickness (mm) 0.5 0.5 0.5 Molecular weight of 1500 1500 1000 PTMG E″ (×10⁷ Pa) 1.80 3.51 6.26 G″ (×10⁷ Pa) 1.02 1.42 3.00 E″/G″ 1.76 2.47 2.09 Ball Amount of compressive 2.40 2.40 2.40 deformation (mm) Ha2-Ha1 (maximum value) 2.0 2.0 2.0 Hb2-Hb1 (maximum value) 9.0 9.0 9.0 W #1 Ball speed (m/s) 73.9 73.9 73.9 Spin (rpm) 2370 2400 2450 Flight distance (m) 249.5 249.2 248.9 SW Spin Dry (rpm) 6710 6700 6760 Spin Wet (rpm) 4900 5000 5100 Feel at impact A A A

TABLE 8 Results of Evaluation Compa. Example 7 Example 8 Example 1 Center Composition (8) (6) (2) Crosslinking temperature 170 170 170 (° C.) Crosslinking time (min) 15 15 20 Diameter (mm) 15 15 39.7 Envelope Composition (3) (7) — layer Crosslinking temperature 170 170 — (° C.) Crosslinking time (min) 20 20 — Core Diameter (mm) 39.7 39.7 39.7 Volume ratio (%) 80.4 80.4 80.4 Central hardness Ho 54 62 70 (JIS-C) Surface hardness He 88 87 86 (JIS-C) Hardness distribution FIG. 5 FIG. 6 FIG. 7 Mid Composition (a) (a) (a) layer Hardness (JIS-C) 94 94 94 Thickness (mm) 1.0 1.0 1.0 Cover Composition (b) (b) (b) Hardness (JIS-C) 47 47 47 Thickness (mm) 0.5 0.5 0.5 Molecular weight of 1800 1800 1800 PTMG E″ (×10⁷ Pa) 3.24 3.24 3.24 G″ (×10⁷ Pa) 2.77 2.77 2.77 E″/G″ 1.17 1.17 1.17 Ball Amount of compressive 2.50 2.40 2.40 deformation (mm) Ha2-Ha1 (maximum value) 4.0 2.0 2.4 Hb2-Hb1 (maximum value) 8.0 6.0 0 W #1 Ball speed (m/s) 73.8 73.9 74.0 Spin (rpm) 2390 2480 2580 Flight distance (m) 249.2 248.0 247.0 SW Spin Dry (rpm) 6680 6740 6750 Spin Wet (rpm) 4480 4510 4550 Feel at impact A A A

TABLE 9 Results of Evaluation Compa. Compa. Compa. Compa. Exam- Exam- Exam- Exam- ple 2 ple 3 ple 4 ple 5 Center Composition (1) (1) (1) (9) Crosslinking 170 170 170 170 temperature (° C.) Crosslinking time 15 15 15 15 (min) Diameter (mm) 25 15 15 15 Envelope Composition (4) (5) (5) (3) layer Crosslinking 170 170 170 170 temperature (° C.) Crosslinking time 20 20 20 20 (min) Core Diameter (mm) 39.1 38.5 39.7 39.7 Volume ratio (%) 76.8 73.3 80.4 80.4 Central hardness Ho 60 60 60 54 (JIS-C) Surface hardness He 90 86 88 88 (JIS-C) Hardness distribution FIG. 8 FIG. 9 FIG. 9 FIG. 10 Mid Composition (a) (a) (a) (a) layer Hardness (JIS-C) 94 94 94 94 Thickness (mm) 1.0 1.6 1.0 1.0 Cover Composition (b) (b) (d) (b) Hardness (JIS-C) 47 47 67 47 Thickness (mm) 0.8 0.5 0.5 0.5 Molecular weight of 1800 1800 1800 1800 PTMG E″ (×10⁷ Pa) 3.24 3.24 7.57 3.24 G″ (×10⁷ Pa) 2.77 2.77 4.01 2.77 E″/G″ 1.17 1.17 1.89 1.17 Ball Amount of 2.40 2.40 2.40 2.40 compressive deformation (mm) Ha2-Ha1 (maximum value) 1.6 2.0 2.0 7.0 Hb2-Hb1 (maximum value) 11 7 7 9.0 W #1 Ball speed (m/s) 73.5 73.6 74.0 73.4 Spin (rpm) 2360 2420 2270 2400 Flight distance (m) 246.0 247.0 251.5 246.8 SW Spin Dry (rpm) 6670 6610 6330 6720 Spin Wet (rpm) 4470 4400 3000 4500 Feel at impact A B C A

As shown in Tables 6 to 9, the golf balls according to Examples are excellent in various performance characteristics. From the results of evaluation, advantages of the present invention are clear.

The above description is merely for illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

1. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, wherein the core comprises a center and an envelope layer positioned outside the center, a ratio of a volume of the core to a volume of a phantom sphere of the golf ball is equal to or greater than 76%, a JIS-C hardness Ho of the cover is less than JIS-C hardness Ho at a central point of the core, at all points Pa included in a zone A that extends over a distance range from equal to or greater than 1 mm to less than 5 mm from a central point of the core, the following mathematical formula (I) is satisfied, at any of points Pb included in a zone B that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point of the core, the following mathematical formula (II) is satisfied, Ha2−Ha1<5   (I), Hb2−Hb1≧5   (II), in the mathematical formula (I), Hal indicates a JIS-C hardness at a point Pa1 that is located radially inward of each point Pa at a distance of 1 mm from the point Pa, and Ha2 indicates a JIS-C hardness at a point Pa2 that is located radially outward of the point Pa at a distance of 1 mm from the point Pa, and in the mathematical formula (II), Hb1 indicates a JIS-C hardness at a point Pb1 that is located radially inward of the point Pb at a distance of 1 mm from the point Pb, and Hb2 indicates a JIS-C hardness at a point Pb2 that is located radially outward of the point Pb at a distance of 1 mm from the point Pb.
 2. The golf ball according to claim 1, wherein the JIS-C hardness Hc of the cover is equal to or less than
 65. 3. The golf ball according to claim 1, wherein a thickness of the cover is equal to or less than 0.8 mm.
 4. The golf ball according to claim 1, wherein a JIS-C hardness Hm of the mid layer is equal to or greater than
 90. 5. The golf ball according to claim 1, wherein a thickness of the mid layer is equal to or less than 1.5 mm.
 6. The golf ball according to claim 1, wherein the cover is formed from a resin composition, and a principal component of a base material of the resin composition is a thermoplastic polyurethane.
 7. The golf ball according to claim 6, wherein a shear loss elastic modulus G″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., is equal to or less than 1.95×10⁷ Pa, and a ratio (E″/G″) of a tensile loss elastic modulus E″ of the resin composition, which is measured under conditions of a vibration frequency of 10 Hz and a temperature of 0° C., to the shear loss elastic modulus G″ is equal to or greater than 1.76.
 8. The golf ball according to claim 7, wherein the tensile loss elastic modulus E″ is equal to or greater 2.00×10⁷ Pa.
 9. The golf ball according to claim 6, wherein a polyol component of the thermoplastic polyurethane is polytetramethylene ether glycol having a number average molecular weight of 1500 or less.
 10. The golf ball according to claim 1, wherein a difference between a JIS-C hardness He at a surface of the core and the hardness Hb2 is equal to or greater than
 10. 11. The golf ball according to claim 1, wherein a difference between a JIS-C hardness He at a surface of the core and the hardness Ho is equal to or less than
 40. 12. The golf ball according to claim 1, wherein the hardness Ho is equal to or greater than 40 but equal to or less than
 80. 13. The golf ball according to claim 1, wherein a JIS-C hardness He at a surface of the core is equal to or greater than 75 but equal to or less than
 95. 14. The golf ball according to claim 1, wherein a difference between a JIS-C hardness He at a surface of the core and a JIS-C hardness Hi at an innermost portion of the envelope layer is equal to or greater than 10 but equal to or less than
 25. 15. The golf ball according to claim 1, wherein a thickness of the envelope layer is equal to or greater than 8 mm but equal to or less than 18 mm.
 16. The golf ball according to claim 1, wherein a diameter of the center is equal to or greater than 10 mm but equal to or less than 20 mm.
 17. The golf ball according to claim 1, wherein a JIS-C hardness He at a surface of the core is greater than a JIS-C hardness at a surface of the center, and the hardness Hm of the mid layer is greater than the hardness He.
 18. The golf ball according to claim 1, wherein a difference between the hardness Ho and the hardness Hc is equal to or greater than
 3. 